1. Field of the Invention
This invention relates generally to telephone party line systems wherein a plurality of individual telephone subscribers utilize the same telephone subscriber line, and more particularly to an improved telephone party line system which provides privacy of communication to each party line subscriber on all calls.
2. Description of the Prior Art
In many rural or other low user density environments, it is most economically advantageous to provide telephone party line systems in which two or more telephone subscribers share a single subscriber line. Such party line arrangements in the past have suffered from numerous disadvantages, such as lack of privacy among the telephone subscribers sharing the party line, the requirement for coded ringing, special calling procedures for reverting calls i.e. calls to another party on the line to prevent ring trip from occuring due to calling party being off hook.
Generally, telephone party line systems of the prior art operate on the basis of polarity and ground dependent signalling over one or two wires from the local or central telephone exchange which establish current paths to ground that are detected at the local or central telephone office for identification of the called party. Privacy among telephone subscribers sharing a party line in such a prior art system is achieved by dc polarity blockage which is commanded from the telephone exchange depending upon which party sharing the line is initiating or terminating a particular telephone call. The ringer at the subscriber set is activated by applying a ringing current between either one or both of the wires on the telephone line and ground. Other known arrangements of the prior art for applying ringing current across the ring and tip lines make use of a set of frequency selective ringers to identify the called party.
Examples of party line systems of the known prior art are illustrated by U.S. Pat. No. 3,668,050 "Telephone Set Identification System Using Reverse Polarity Interrogation Signal" which describes a telephone identification system in which telephones are equipped with circuits that are responsive to a reversal of the battery voltage for the central office to generate and transmit a preselected identification signal to the central office. The frequency of the identification signal is determined by connecting either a resistor-capacitor series circuit or a normally closed switch across a selected tap switch. U.S. Pat. No. 4,221,935 "Control Circuit for Telephone Subscriber Station" uses a microprocessor and external logic to provide automatic number identification, party line privacy, fully selected ringing, remote station verification and disconnect, and reverting calling. U.S. Pat. No. 4,149,040 "Identifier Circuit for Identifying Party Line Subscribers" describes an identifier circuit for identifying party line subscribers in which the line adaptors have a pair of terminals which may be selectively connected to ground. U.S. Pat. No. 3,035,123 "Party Line Telephone Circuit" describes a privacy system in a two-party line telephone circuit and also relies on the charging and discharging of a capacitor. U.S. Pat. No. 3,530,253 "Party Line Circuit Using Diodes in Ringing Control, Lockout and Revertive Calling" describes a party line system for secrecy and reverting calling in two-party stations. U.S. Pat. No. 4,293,737 "Ringing Decoder Circuit" describes a ringing decoder circuit for automatically ringing a single party on a multiparty telephone line. U.S. Pat. No. 3,297,270 "Automatic Number Identification Having Four-Party Detection" describes an automatic number identification system.
The aforementioned prior art references and other known party line communication systems of the prior art are generally unable to provide complete privacy between subscribers when more than two subscribers share the same party line. A further disadvantage of prior art party line systems is the requirement of special installation and ground signalling for identification of a called party with the associated requirement for specialized and complex circuitry at the telephone central office. The prior art technique of using frequency selective ringing circuitry for identification of the called party results in reliability, cost and maintainence problems which are eliminated by applicants' invention. Such prior art systems are particularly undesirable when reverting calls are initiated, i.e. a call between one subscriber on the party line to another subscriber on the same party line, since such reverting calls using known systems of the prior art require cumbersome dialing schemes having coded ringing to prevent parties other than the called party from being rung.